1. Field of the Invention
The present invention is directed to a vacuum housing for an electron tube having comprising an anode and a cathode, the vacuum housing being of the type having a volume containing a space for the acceptance of the anode that is connected via a shaft-shaped housing section to a chamber for the acceptance of the cathode, with the chamber and the volume for the acceptance of the anode being connected to one another via a weld.
2. Description of the Prior Art
In an X-ray tube with such a vacuum housing, it is possible to arrange means for the deflection of the electron beam, which propagates through the shaft-shaped housing section during the operation of the X-ray tube, in the region of the shaft-shaped housing section, and, thus close to the electron beam to be deflected.
The capability of deflecting of the electron beam and thus also deflecting the focal spot, is especially important in the context of computed tomography since this allows an improvement of the image quality by the known measure known of displacing the focal spot between two limit positions, thereby doubling the data available for the computation of the image of a body slice.
When, during the operation of the X-ray tube of the type initially described, the vacuum housing the X-ray tube lies at a more positive potential than the cathode which is for example, at ground potential, a large portion of the electrons scattered back from the anode (secondary electron bombardment) is captured by the shaft-shaped housing section and at the adjacent regions of the vacuum housing. Apart from its purpose as a housing, the vacuum housing therefore also performs the function of a diaphragm that serves to decrease the extra-focal radiation, especially in the region of the shaft-shaped housing section.
An X-ray tube with a vacuum housing of the type initially described is disclosed in European Application 0 460 421. The shaft-shaped housing section in this vacuum housing is bonded by a weld to an approximately planar wall of the volume of the vacuum housing containing the space for the acceptance of the anode. This weld is conventionally made by TIG-welding and does not always resist the thermomechanical stress to which it is exposed as a result of the aforementioned secondary electron bombardment. Total failures of X-ray tubes therefore frequently occur as a result of inadequate vacuum tightness of the vacuum housing.